1. Field of the Invention
The present invention relates to a VP (Virtual Path) protection system for use when an error occurs on an Asynchronous Transfer Mode (ATM) network, and more particularly to a VP protection system and a VP protection method for restoring many VPs at a time when errors occur in many VPs on the same link at the same time.
2. Description of the Related Art
When errors occur on a plurality of working VPs on an working (active) path at the same time, many control messages, including error messages and switching request messages, are generated for VPs. This results in heavy message-forwarding traffic among nodes in the switching section of the network.
To solve this problem, a, method for grouping a plurality of VPs into a Virtual Path Group (VPG) is known. For example, Japanese Patent Laid-Open Publication No. A-10-84362 discloses a method for providing an address table associating VPGs with VPs belonging thereto. In addition, ITU-T SG13 Recommendation Draft I. ps (issued in November, 1997) proposes OAM (Operation and Maintenance) cells for use as switching control messages for each VPG.
The conventional VP protection method will be described with reference to drawings. FIG. 4 is a diagram schematically showing an example of an ATM network. This network is composed of a plurality of ATM nodes 101-104. ATM nodes 101 and 102 are linked by physical link 111, ATM nodes 102 and 103 are linked by physical link 112, ATM nodes 101 and 104 are linked by physical link 113, and ATM nodes 104 and 103 are linked by physical link 114, respectively.
n working VPs passing through the ATM node 102 are set up between the ATM nodes 103 and 101 which are the endpoint nodes of a switching section on this network. These n VPs are grouped into an working VPG 121. For the working n VPs between the ATM nodes 103 and 101 which are the endpoint nodes of the switching section, n standby (protection) VPs for which only a path is determined but no bandwidth is assigned are provided in advance. These n standby VPs are grouped into a standby VPG 122.
Also, in the network configuration shown in FIG. 4, two message exchange channels are defined between the ATM node 101 and the ATM node 103: one is a message exchange channel 123 from the ATM node 101 to the ATM node 103 and the other is a message exchange channel 124 from the ATM node 103 to the ATM node 101.
If an error occurs on a physical link 111 in the configuration described above the ATM node 101 senses this error and sends a VPG switching request message 131 via the message exchange channel 123.
Because only paths are set up but no bandwidth is allocated to the standby VPs of the standby VPG 122, the nodes on the standby path via which the switching request message 131 is sent allocate bandwidth required by the standby VPs.
The switching request message 131 is sent from the ATM node 101 which is the switching-request message originating endpoint to the ATM node 103 which is the switching-request message receiving endpoint. During this period, the ATM node 101 and the intermediate ATM node 104 allocate bandwidth to the standby VPs.
After allocating bandwidth at the ATM node 103, a VPG switching-response message 132 is sent through the message exchange channel 124 to the ATM node 101 via the intermediate ATM node 104 to notify that bandwidth has been allocated at all nodes on the standby path.
Upon receiving the switching-response message 132, the ATM node 101 knows that the bandwidth resource has been allocated to the standby VPG. Then, the ATM node 101 switches all working VPs to standby VPs.
Referring to FIG. 5, the internal processing of a node during the above-mentioned protection processing will be described. FIG. 5 shows the configuration of, and processing flow of control within, an intermediate node .(ATM node 104 in FIG. 4) on a standby path when conventional protection is performed with the use of VPG switching messages.
As shown in FIG. 5, an ATM node 200 comprises a VPG switching message receiving circuit 201 which receives grouped alarm messages or switching request messages, a VPG switching message sending circuit 202 which sends a VPG switching message to the next node, a VPG expansion circuit 203 which expands a VPG into individual VPs, a VP resource allocation circuit 204 which allocates resources to each VP, one or more node resource management circuits 206-i (i=1, 2, . . . , n) which manage the allocation of resources of the entire node, a VP resource information management table 251 which stores therein the resource requirements for each VP, and a VPG configuration information management table 252 which stores therein information for identifying the VPs constituting the VPG.
The ATM node 200 forward,s a switching-request message 291 or a switching-response message 292. When the switching-request message 291 arrives at the ATM node 200, the VPG switching message receiving circuit 201 receives it, identifies that the message is issued to the VPG, and sends it to the VPG expansion circuit 203.
The VPG expansion circuit 203 references the VPG configuration information management table 252 to expand the VPG specified by the switching-request message into the individual VPs constituting the VPG and then sends the identifiers of the individual VPs to the VP resource allocation circuit 204.
The VP resource allocation circuit 204 finds the amount of resources required for each VP by referencing the VP resource information management table 251 and allocates the required node resources and network resources with the use of the node resource management circuits 206-i (i=1, 2, . . . n).
After allocating resources to all VPs, the VP resource allocation circuit 204 sends the switching-request message 291, either via the VPG expansion circuit 203 or directly, to the VPG switching message sending circuit 202. The VPG switching message sending circuit 202 sends the switching-request message 291 to the next node.
The VPG switching-response message 292 is sent or received by the VPG switching message receiving circuit 201 and the VPG switching message sending circuit 202.
Although the configuration of an intermediate node is shown in FIG. 5, it may be applied also to an endpoint node. For example, at a switching-request message sending endpoint node (ATM node 101 in FIG. 4), the switching-request message 291 shown in FIG. 5 is an error information message and the switching-response message is terminated at this node.
Also, a switching-request message receiving endpoint node (ATM node 103 in FIG. 1) sends a switching-response message instead of a switching-request message. The operation of this endpoint node is basically the same as that of the intermediate node shown in FIG. 5.
As described above, the conventional VP protection method uses VPG node-to-node messages to reduce the amount of transfer messages. However, after receiving a message, the endpoint nodes and intermediate nodes that forward the VPG message must allocate resources, such as bandwidths and paths, to each VP of the VPG where an error occurred.
This requires the node to expand the VPG message into messages for the VPs and, after completion of processing for all VPs, to generate a VPG message again for transmission to the next node.
In this case, the node cannot forward the message to the next node until the processing of all VPs is completed, increasing the time needed for recovery from the error.
This problem will be described with reference to FIG. 6. FIG. 6 shows the switching sequence on the standby path when error recovery is performed with the conventional protection method in the network configuration shown in FIG. 1.
As shown in FIG. 6, when the ATM node 101 detects an error in the working path, it first references the message issued to the VPG and then performs VP expansion processing 311 to expand the VPG into individual VPs.
After performing VP resource allocation processing 312 for each VP, the ATM node 101 sends the VPG switching-request message to the ATM node 104 which is the next node.
The ATM node 104 performs the same processing in the same sequence. That is, it performs the VP expansion processing 311 and the VP resource allocation processing 312 sequentially. After the VP resource allocation processing 312 is completed, the ATM node 104 sends the VPG switching-request message to the ATM node 103 which is the next node.
The ATM node 103, the other endpoint of switching, performs the same processing and sends the switching-response message to the ATM node 101.
The ATM node 104 performs forwarding processing 313 to forward the switching-response message to the ATM node 101. When the switching-response message arrives at the ATM node 101, the switching processing ends.
According to the conventional method, each node has to expand the VPG and allocate resources for each VP before sending the switching-request message to the next node, as described above. Therefore, the message transmission wait time increases as the number of VPs increases, resulting in a significant increase in the error recovery time.
The present invention seeks to solve the problems associated with the prior art described above. It is an object of the present invention to provide a VP protection system and a VP protection method, for use when an error occurs on an ATM network with many VPs, which are capable of sending a message quickly to other nodes on the ATM network to reduce the time required for error recovery.
To achieve the above object, according to one aspect of the present invention, there is provided a VP protection system which restores VPs when an error occurs on an ATM (Asynchronous Transfer Mode) network, wherein a predetermined node in the ATM network allocates resource information on a plurality of working VP groups for management of resources on a group basis and wherein, when a plurality of VPs must be switched to standby VPs, which are previously-reserved as bypass paths, due to the error that occurred in the plurality of VPs in a link at the same time, at least an intermediate node allocates the resources for the group composed of the plurality of VPs in response to a grouped VP switching request message and forwards the grouped VP switching request message to a next node.
According to another aspect of the present invention, there is provided a VP protection system, for use on an ATM network including a plurality of ATM nodes performing protection, a plurality of working VPs (virtual paths) usually used as information transmission paths among the plurality of nodes, and a plurality of standby VPs used as bypass paths when an error occurs on the working VPs, wherein, when the plurality of working VPs must be switched to the corresponding standby VPs upon detection of the error on the plurality of working VPs, messages are exchanged among the ATM nodes with the plurality of VPs as a group, each of the ATM node comprising a VPG resource information management table which stores therein an amount of resource requirements for each virtual path group (called VPG); a VP resource information management table which stores therein the amount of resource requirements for each VP; a VPG configuration information management table which stores therein information for identifying the VPs constituting the VPG; a VPG switching message receiving circuit and a VPG switching message sending circuit which receive and send a VPG switching request message or a VPG switching response messages transferred between endpoint nodes in a switching section; one or more node resource management circuits which allocate node resources or network resources in response to a request generated in the node; a VPG resource allocation circuit which references the VPG resource information management table to allocate the node resources and the network resources to the VPG at a time in response to the VPG switching request message; a VP expansion circuit which expands the VPG into the individual VPs constituting the VPG; and a VP resource distribution circuit which references the VP resource information management table and distributes the VPG resources allocated by the VPG resource allocation circuit to the individual VPs expanded by the VP expansion circuit.
According to another aspect of the present invention, there is provided a VP protection system wherein, immediately after the VPG resource allocation circuit has allocated resources of the VP group, the VPG switching message sending circuit forwards the message to the next node and, at the same time, the VP expansion circuit expands the VP group into VPs and the VP resource distribution circuit distributes the resources to the VPs.
According to another aspect of the present invention, there is provided a VP protection system, wherein, after the VPG switching message receiving circuit receives the switching response message, the VPG switching message sending circuit forwards the switching response message to the next node without checking that the resource distribution circuit has completed resource distribution to the VPs in the ATM node.
According to another aspect of the present invention, there is provided a VP protection system, wherein the node further comprises a resource allocation checking circuit to receive a resource allocation completion notification from the resource distribution circuit upon completion of VP resource distribution, and wherein, in response to the switching response message received by the VPG switching message receiving circuit, the resource allocation checking circuit checks if a resource allocation completion notification is received. If the notification is already received, the VPG switching message sending circuit forwards the switching response message to the next node; if the notification is not yet received, the resource allocation checking circuit waits for the resource allocation completion notification and then the VPG switching message sending circuit forwards the switching response message to the next node.